<!DOCTYPE html>
<html>
    <head>
        <meta charset="UTF-8">
        <meta name="测试3" content="测试3">
        <meta description="测试3" content="测试3">
        <title>测试2</title>
        <style>
           * {
                margin: 0;
                padding: 0;
           }
           .father {
                width: 200px;
                height:200px;
                background-color: blue;
                /* border-top: 1px solid; */
                /* padding-top: 1px; */
                overflow: hidden;
           }
           .son {
                margin-top: 20px;
                width: 100px;
                height: 100px;
                background-color: green;
           }
        </style>
    </head>
    <body>
        <div class="father">
            <div class="son"></div>
        </div>
    </body>
</html>